Hitherto, making of lithographic printing plates has been performed using a system for exposing printing plate materials to light via litho films as intermediate materials. However, with the recent rapid progress of digitization in the field of graphic arts, the process of making printing plates is on its way to shifting to a CTP system that digital data input into a computer and manipulated therein are output directly to printing plate materials. Further, with a view toward ever-more rationalizing the platemaking process, lithographic printing plate materials of the type which are exposed to light and subjected to printing operations without development processing, namely the type which eliminate the need for development processing, are being developed.
As a method of eliminating the processing operation, there is a method referred to as on-press development wherein an exposed printing plate material is mounted on the plate cylinder of a printing press and thereto dampening water and ink are supplied as the plate cylinder is rotated: as a result, removal of the unexposed area of the image-forming layer is effected on the press. More specifically, this method is a manner of platemaking in which after exposure the printing plate material is mounted on a printing press as it is and development thereof is completed in the process of a usual printing operation. Lithographic printing plate materials suitable for such on-press development are required to have image-forming layers soluble in dampening water and ink solvents, and besides, it is advantageous that the plate materials are sensitive to infrared laser because they are developed on a printing press placed in an illuminated room, and so required to have illuminated room handling suitability.
As a lithographic printing plate material capable of the on-press development, for instance, Japanese Patent No. 2,938,397 discloses the heat-sensitive lithographic printing plate having a hydrophilic support provided with a hydrophilic image-forming layer containing fine particles of a thermoplastic hydrophobic polymer dispersed in a hydrophilic binder polymer. In such a heat-sensitive lithographic printing plate, the fine particles of a thermoplastic hydrophobic polymer are fused and coalesced by heating caused upon exposure to infrared laser; as a result, a lipophilic imaging area is formed. When the resultant plate is mounted on the plate cylinder of a printing press as it is and a printing operation is started, the unexposed area is removed by dampening water and/or ink to begin with, namely on-press development is performed, and prints of good quality are obtained by further continuation of the printing operation.
In addition, JP-A-2001-277740 discloses the on-press developable heat-sensitive lithographic printing plate whose press life is improved by use of thermally reactive compound-enclosed microcapsules.
Further, JP-A-2002-29162 discloses that a satisfactory press life can be attained with the on-press developable heat-sensitive lithographic printing plate having an image-forming layer containing microcapsules in which a vinyloxy group-containing compound is enclosed, a hydrophilic resin and an acid precursor.
Furthermore, JP-A-2002-46361 discloses that a satisfactory press life can be attained with the on-press developable heat-sensitive lithographic printing plate having an image-forming layer containing microcapsules in which an epoxy group-containing compound is enclosed, a hydrophilic resin and an acid precursor.
Additionally, JP-A-2002-137562 discloses that a satisfactory press life can be attained with the on-press developable heat-sensitive lithographic printing plate having an image-forming layer containing microcapsules in which a radical-polymerizable group containing compound is enclosed, a hydrophilic resin and a heat-sensitive radical generator.